1987] 
Uetz & Hart sock — Micrathena gracilis 
113 
day). The spider may therefore reap this collective caloric benefit with- 
out actively attacking each small insect striking its web during the day. 
By following the predatory sequence of Micrathena gracilis (Fig. 
2), it is clear that throughout the sequence, the spider is presented 
with many more small insects than large insects. After the web has 
restrained an insect, the spider will either attack it or ignore it. It has 
been shown that the spider attacks insects > 3 mm with a significantly 
higher frequency than it does those < 3 mm, which are ignored most 
of the time. Similar results were seen in Micrathena schreibersi, a 
neotropical congener (Shelly 1984). The term “ignore” implies that 
the spider actually makes a choice of what it attacks, which is what is 
implied in this study, and has been used by other arachnologists (e.g. 
Olive 1980; Riechert and Luczak 1982; Shelly 1984). During the 
course of observations, spiders did appear to make a choice of 
whether or not to attack certain insects, most probably based on 
some sort of vibrational stimuli. For example, the spider was often 
seen orienting toward the impact area of a small insect with the web, 
plucking the radii in that area, and even advancing a few millimeters 
toward the insect, but not attacking it. The impact of the insect may 
have been enough of a vibrational stimulus to initiate the attack 
sequence, but the lack of vibration after impact indicated either a very 
small prey item or no prey item. Suter (1978) found that the impact 
stimulus was important in initiating the attack behavior of Cyclosa 
turbinata, another forest-dwelling orb-weaver. This particular spe- 
cies may be capable of detecting the mass of an insect by its impact 
vibration, which could be, according to Suter, a mechanism for 
discerning the insect’s relative food value. This same mechanism 
could be operating in Micrathena. 
Riechert and Luczak (1981) suggest that three parameters regard- 
ing profitability should be important in the spider’s decision to con- 
sume (attack) or ignore the prey item: prey type, prey size, and level of 
hunger. Observations of other species indicate that orbweavers are 
able to discriminate between prey and alter their attack behavior 
accordingly (Eberhard 1967; Robinson 1969; Robinson, Mirick, and 
Turner 1969; Robinson and Olazarri 1971; Robinson and Mirick 
1971; Robinson & Robinson 1973, 1976) allowing them to secure 
prey in such a way as to prevent quick escapes and/or minimize the 
risk of injury. Most of the potential prey of Micrathena is small 
Hymenoptera and Diptera (as seen in availability estimates in Table 
1), and they pose little threat to the spiders. Micrathena exhibits the 
more primitive “bite-wrap” behavior pattern (Robinson et al. 1969), 
